Model Calculations for Io's Atmosphere at Eastern and Western Elongations

Abstract

Numerical calculations are performed for Io's sublimation atmosphere at eastern and western elongations using an improved version of the multispecies hydrodynamic model first developed by Wong and Johnson (1996). Subsolar SO 2 frost temperatures of 113 and 120 K are adopted to cover a range of plausible atmospheric abundances that are consistent with recent observations. In the model, the incoming plasma ions are allowed to reaccelerate to the unperturbed corotational speed after colliding with a neutral in the atmosphere above the ion gyropause where the local ion gyration frequency is higher than the neutral collision frequency. This reenergization of ions gives an energy deposition rate per ion that can be up to 2.5 times its upstream corotational energy rate, depending on the local atmospheric column. It is found that, in general, SO 2 is the dominant species in the dayside atmosphere while the noncondensibles, O 2 and SO, are the dominant species in the nightside atmosphere. The gas-phase reactions among these noncondensibles can produce a substantial amount of SO 2 in the nightside atmosphere. In all four cases considered here (high and low density; eastern and western elongations) we find that there exists a global exobase above all of the surface and that the ratio of dayside to nightside average SO 2 abundance is surprisingly limited to the range 17–95, even though surface vapor pressure on the nightside is many orders of magnitude lower. The dayside [SO]/[SO 2] mixing ratio is ∼3.2–7.1%, consistent with recent observations by Lellouch (1996) if SO is colocated with SO 2. We have also included simple NaX chemistry in the model and found that atomic sodium is far more abundant than molecular sodium because of fast reactions of NaX with O. Depending on the solar zenith angle and Io's orbital location, the exobase altitude ranges from 30 to 465 km while its temperature ranges from 220 to 2800 K. It is also found that at western elongation the dayside atmospheric flow is enhanced by the impinging plasma ions, resulting in an overall hotter and bigger atmosphere which has an average SO 2 column that is about twice as high as what would result from a hydrostatic atmosphere in vapor pressure equilibrium with the surface. At eastern elongation, the plasma energy is added to the nightside atmosphere, increasing its pressure and causing the formation of a standing oblique shock that occurs farther upstream in the day-to-night gas flow than in the case for western elongation. Consequently, the dayside atmospheric temperature is lower, and the day-to-night transport rate is reduced, which results in lower relative abundance for all other species except SO. The results from the high density case at western elongation give a line-of-sight tangential column density at the terminator for S, O, and Na, respectively, of 1.7×10 15, 2.5×10 15, and 4.3×10 13 cm −2. These profiles above the surface are reasonably consistent with those in Io's corona inferred from recent HST measurements for S and O (F. Roesler 1999, private communication) and from eclipse measurements for Na (Schneider et al. 1991a).